496 Horace B. Woodicard — The Chalky Boulder-clay. 



Judging by facts, we may believe that these cakes and boulders 

 were in some cases dislodged from outliers frozen to the base of the 

 land-ice, and shifted to higher levels in the ice along planes produced 

 by its movement over an irregular surface. Cliffs or escarpments, as 

 well as outliers, locally formed impediments to the progi*ess of ice. 



As is well known, the Chalk in Norfolk, at Whitlingham, Trowse, 

 and Trimingham, was markedly disturbed, and the evidence brought 

 forward by Mr. Clement Eeid and myself shows that great force 

 must have been exerted, sufficient to bend the Trimingham Chalk 

 into folds, and to uptilt the Trowse Chalk, whereby we see stages 

 in the formation of the huge cakes of Chalk elsewhere disrupted 

 and incorporated with the Drift. Cliffs that had been formed 

 in the Chalk along Pliocene coast-lines may then have been effaced. 



It is, of course, by no means improbable that some of the large 

 masses of rock may have slipped from scarps on to ice when 

 the ice-sheet was being temporarily melted back, and they may 

 thereby have become incorporated into the subsequently thickened 

 moving ice.^ 



At Hendon and Finchley masses of reconstructed London Clay 

 have been incorporated in the gravelly Drifts, and these may have 

 been torn from the parent strata during an early stage of the period 

 of maximum glaciation.^ 



If we believe that the main mass of Boulder-clay was deposited 

 on the melting back of the ice - sheet, or from the melting 

 of basal portions of the ice, and that there were alternations 

 in the climate so that the ice extended and receded for some 

 distance more than once before it finally retreated, we can under- 

 stand the intercalation of water-distributed sands and gravels, as 

 well as laminated muds, with Boulder-clay, and more especially 

 along the margin of the glaciated area. Such instances would, no 

 doubt, follow the final retreat of the ice, and we have indeed 

 examples at Wymondham, in Norfolk, of fine sand and coarse 

 gravel interbedded with Chalky Boulder-clay. Instances are more 

 frequent towards the southern area. Mr. Whitaker long ago drew 

 attention to the intercalation of Boulder-clay in the sands and 

 gravels of Hertfordshire, and there are fine examples at Tingewick, 

 Eadcliff, and elsewhere, near Buckingham, where coarse boulder- 

 gravel with large blocks of Jurassic and other rocks, and finer 

 gravel and sand, occur in association with the Boulder-clay. 



The extensive tracts of gravel and loam on the Chalk areas of 

 Buckinghamshire, in the vicinity of Chesham, Amersham, and 

 Eickmansworth, may to some extent have resulted from the melting 

 of ice, and the more or less torrential distribution of material, 

 including large greywethers. Some beds of gravel were doubtless 

 distributed during the earlier stages of the period of maximum 

 glaciation, and they would yield an abundance of Jurassic 



1 See also T. M. Keade, Quart. Journ. Geol. Soc, vol. xxxviii, p. 232, and 

 G. A. J. Cole, Geol. Mag. 1895, p. 563. 



^ See discussion on paper by Dr. Hicks, Quart. Journ. Geol. Soc, vol. xlvii, 

 p. 583. 



